1. Field of the Invention
This invention relates to a high-temperature gasket, and more particularly to a gasket exhibiting improved sealability and heat resistance under temperature condition exceeding 950.degree. C. for using in an engine of an automobile or the like and consisting mainly of ceramic instead of asbestos.
2. Related Art Statement
In general, the high-temperature gasket used in the above application is composed mainly of asbestos and contains an elastic substance for giving an elasticity such as natural rubber (NR), acrylonitrilebutadiene rubber (NBR), styrene butadiene rubber (SBR) or the like and an additive such as a vulcanizing agent or the like, if necessary.
As such a gasket, there were "joint sheet" like gasket obtained by heating and shaping the asbestos and the elastic substance, and "beater sheet" like gasket obtained by shaping an emulsion of asbestos and elastic substance or a latex-like slurry.
Since 60-95% by weight of asbestos fiber is included in these known gaskets, such asbestos fibers are scattered not only at the production step of the gasket but also from the working equipment to cause a fear of health impediment, so that it is demanded to improve the handling of asbestos.
In Japan, the handling of asbestos is regulated by various laws accompanied with the signing of Occupational Cancer Convention on 1974 up to the present. For ins,tance, there are "Rule for Prevention of Impediment through Specified Chemical Substances" enacted on Apr. 1, 1976, and "Lung Cancer or Skin Tumor through Works exposed to Asbestos Atmosphere" in a part of the Labor Standard Rules revised on Mar. 30, 1978 and on cancer origin substance, cancer origin factor or diseases through works at cancer origin step, and the like.
Further, EPA (Environment Protection Agency) in USA proposed the prohibition on the use of asbestos articles on January, 1983, and the use prohibition on building materials on January, 1988 after the comprehension at OMB (Office of Management and Budget), and proposes the complete prohibition on use from 1991.
In this connection, three big gasket makers in USA declared to stepwisedly receive such a proposal on July, 1986.
Besides the above two contries, "Treaty and Recommendation on Safety in Utilization of Asbestos" are internationally discussed and considered in accordance with the proposal of ILO (International Labor Organization) on June, 1986.
Under such an international situation, it is recently attempted to produce gaskets by using inorganic fibers such as sepiolite fiber, glass fiber, carbon fiber, stainless fiber and the like, or organic fibers such as aromatic polyamide fiber, phenolic fiber, polyethylene fiber and the like instead of asbestos fiber.
As mentioned above, the gaskets using the asbestos fiber not only hold pollution problem but also have a drawback that the degradation of strength is caused by dehydration of crystal water at a high temperature of, for example, not lower than 950.degree. C. though good properties are maintained near to 600.degree. C.
On the other hand, the gaskets using the inorganic or organic fibers instead of the asbestos fiber have the following drawbacks.
That is, the inorganic fiber such as sepiolite or the like has crystal water, so that the degradation of strength is caused in a high temperature region likewise the asbestos. Furthermore, when the gasket is produced from the inorganic fiber alone, there is a problem that the tensile strength and the like are poor likewise the gasket using the asbestos fiber. Particularly, the carbon fiber among the inorganic fibers and the organic fibers such as aromatic polyamide fiber, phenolic fiber, polyethylene fiber and the like considerably degrade the fiber properties from about 200.degree. C. and completely carbonize or burn above 400.degree. C. to cause the bonding degradiation, so that they can not be used at a high temperature above 400.degree. C.